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The fundamentals of operation of
Domain Name System servers, as explained in the preceding topics in
this section, are specified in the main DNS standards, RFC 1034 and
1035. These documents are pretty old by computer industry standards;
they were published in 1987. To the credit of the designers of DNS,
most of what they originally put into the DNS protocol is still valid
and in use today. The creators of DNS knew that it had to be able to
scale to a large size, and the system has in fact successfully
handled the expansion of the Internet to a degree far beyond what anyone
could have imagined 15 or so years ago.

As originally defined, the Domain
Name System requires that DNS information be updated manually by editing
master files on the primary server for a zone. The zone is then copied
in its entirety to slave servers using a polling/zone-transfer
mechanism. This method is satisfactory
when the internetwork is relatively small, and changes to a zone are
made infrequently.

However, in the modern Internet,
large zones may require nearly-constant changes to their resource records.
Hand-editing and constantly copying master files can be impractical,
especially when they grow large, and having slave servers get out of
date between zone transfers may lead to reliability and performance
concerns. For these reasons, several enhancements to the operation of
DNS servers have been proposed over the years. I'm going to take a closer
look at three of them here.

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